The invention relates to an electrochemical cell comprising a negative electrode, the electrochemically active material of which consists of an intermetallic compound forming a hydride with hydrogen, which compound has the CaCu.sub.5 -structure and the compositional formula AB.sub.m C.sub.n, where m+n is between 4.8 and 5.4, where n is between 0.05 and 0.6, in which A consists of Mischmetall or of one or more elements selected from the group consisting of Y, Ti, Hf, Zr, Ca, Th, La and the remaining rare earth metals, in which the total atomic quantities of the elements Y, Ti, Hf and Zr may not be more than 40% of A, in which B consists of two or more elements selected from the group formed by Ni, Co, Cu, Fe and Mn, where the maximum atomic quantity per gram atom of A is for Ni: 3.5, for Co: 3.5, for Cu: 3.5, for Fe: 2.0 and for Mn: 1.0, and in which C consists of one or more elements selected from the group formed by Al, Cr and Si in the indicated atomic quantities: Al: 0.05-0.6, Cr: 0.05-0.5 and Si: 0.05-0.5.
The cell may be in open communication with the atmosphere or may be sealed from the atmosphere. A cell sealed from the atmosphere may have a valve which is proportioned so as to become operative at a previously fixed pressure.
In a rechargeable cell of the closed type the electrochemically active part of the positive electrode consists, for example, of nickel hydroxide, silver oxide or manganese oxide, nickel hydroxide being generally preferred for practical reasons.
In the cell an electrolyte is used which generally consists of an aqueous solution of one or more alkali metal hydroxides, such as lithium hydroxide, sodium hydroxide and potassium hydroxide, having a pH exceeding 7.
The cell may furthermore comprise a separator which separates the electrodes electrically but permits transport of ions and gas. The separator may consist of synthetic resin fibres (which may or may not be woven), for example of polyamide fibres or polypropylene fibres.
Such an electrochemical cell is described in U.S. Pat. No. 4,487,817. In this patent the electrochemically active material of the negative electrode is selected so as to be very resistant to corrosion, thereby limiting the deterioration of the electrochemical capacity of the cell to the largest extent possible.
A disadvantage of the electrochemical cell as described in this patent is that in the first charge and discharge cycle the capacity of the cell is below the maximum capacity and that during the first 20 to 30 charge and discharge cycles the capacity increases only gradually to its maximum value. This phenomenon is called "activation".
A further disadvantage of the known electrochemical cell is the relatively low power density of the cell and at a low operational temperature, for example below 0.degree. C. The expression "power density" is to be understood to mean herein the capacity of the cell at high discharge rates. Both properties previously described reduce the usefulness of the stable hydride-forming materials as described in this patent.